Conventionally, radio base stations have used multiplexing techniques so as to assign channels to mobile stations and communicate with the mobile stations. Among such multiplexing techniques, space division multiplex and CDMA (Code Division Multiple Access) are known to effectively use frequency resources.
In CDMA, a transmission power of each mobile station is adjusted by a transmission power control.
In CDMA, all mobile stations use carrier waves having the same frequency. In such circumstances, if all mobile stations transmitted signals using the same transmission power regardless of the distances from a base station, the base station could not separate signals from remote mobile stations due to the strength of the signals from near mobile stations. This problem is called “near-far problem”. The transmission power control is indispensable in preventing the near-far problem.
In the transmission power control, usually, the radio base station diffuses the signals from each mobile station to obtain SIR (Signal to Interference Ratio), namely a ratio of received signal power to interference power, of each mobile station, and controls each mobile station so that the SIR of the mobile station are kept to be a constant value. Refer to, for example, Japanese Laid-Open Patent No. 9-284205 for the transmission power control.
In CDMA, the SIR indicating the communication quality is used as an index during control exercised to properly separate the signals sent from the mobile stations.